Precision resistors are often required in analog CMOS and BiCMOS integrated circuits. Where a conventional double-level polysilicon process is used to fabricate an integrated circuit requiring precision resistors, several competing factors must be considered in determining how the precision resistors will be implemented. These factors include the required accuracy of capacitors in the circuit, the importance of minimizing the size of any non-precision resistors in the circuit, the level of precision the most precise resistors are required to exhibit, and cost.
The competing nature of these factors is apparent in the conventional double-level polysilicon process shown in FIGS. 1-7. FIGS. 1-7 are cross-sectional elevation views showing successive stages in the fabrication of a CMOS device having two polysilicon resistors, each with a different sheet resistance, and a capacitor using a double-level polysilicon process according to the prior art.
Referring to FIG. 1, fabrication of the CMOS device begins with the formation of P- epitaxial layer 12 on P+ substrate 10. P+ substrate 10 has a thickness of about 625 microns and resistivity of about 0.015.OMEGA.-cm. P- epitaxial layer 12 has a thickness of about 20 microns and resistivity of about 6-8.OMEGA.-cm. An oxide layer 14 is formed over P- epitaxial layer 12 and patterned and etched to form a window exposing the surface of epitaxial layer 12 at area 16. A dopant, such as phosphorus, is then implanted with a dose of 3.7E12 atoms/cm.sup.2 at 150 keV in P- epitaxial layer 12 through the window at area 16 to form N well 18. During the diffusion of N well 18, oxide layer 14 grows in the window to form a recessed oxide region at area 16 resulting in the structure shown in FIG. 1.
Oxide layer 14 is then removed and a pad oxide layer (not shown) having a thickness of about 500 Angstroms is formed over P- epitaxial layer 12 and N well 18. A nitride layer (not shown) having a thickness of about 1400 Angstroms is formed over the pad oxide layer. The nitride layer is then patterned using photolithographic techniques and etched to expose areas between moat regions. Conventional Boron and Phosphorous channel stop implants (not shown) may optionally be performed at this point. Thick field oxide regions 20 are then thermally grown to a thickness of about 12000 Angstroms in the exposed areas. The remaining portions of the nitride layer and pad oxide layer are then removed by etching. A gate oxide layer 22 is then thermally grown over the face of epitaxial layer 12 and N well 18 between field oxide regions 20 to a thickness of about 425 Angstroms resulting in the structure of FIG. 2.
A first polysilicon layer having a thickness of about 5000 Angstroms is then deposited over gate oxide layer 22 and field oxide regions 20 and heavily doped with an N type dopant, for example by implanting phosphorus at a dose of 1.6E16 atoms/cm.sup.2 at 60 keV, and annealed at 900 degrees Centigrade for 30 minutes to render its sheet resistance low (about 20.OMEGA. per square). Alternatively, the polysilicon layer could be doped in a furnace using POCl.sub.3 as a dopant source. For example, a deposition time of 10 minutes at 950 degrees Centigrade would give a sheet resistance of 20.OMEGA. per square. The first polysilicon layer is patterned and etched to form transistor gates 24 and lower capacitor plate 26. The low sheet resistance of the first polysilicon layer insures that gates 24 will be of low resistance and that the voltage coefficient of capacitance of lower capacitor plate 26 will be minimized.
An interlevel dielectric layer 27 is then formed over gates 24 and lower capacitor plate 26 to a thickness of about 200-1000 Angstroms. Interlevel dielectric layer 27 may be a formed from either a single grown or deposited oxide layer or from alternating layers of oxide, nitride, and oxide. A second polysilicon layer 28 having a thickness of about 5000 Angstroms is then deposited at 620 degrees Centigrade over interlevel dielectric layer 27 and those portions of gate oxide layer 22 and field oxide regions 20 that are exposed. Second polysilicon layer 28 is lightly doped with an N type dopant to render its sheet resistance high (in the range of about 500-1000 ohms/square), resulting in the structure shown in FIG. 3. This doping may be accomplished, for example, by implanting phosphorous at a dose of 1.7E15 atoms/cm.sup.2 at 60 KeV and then annealing at 900 degrees Centigrade for 30 minutes.
A layer of photoresist is then formed over second polysilicon layer 28 and patterned to form resist mask regions 30a, 30b, and 30c, resulting in the structure shown in FIG. 4. Resist mask regions 30a and 30b define a pair of resistors in second polysilicon layer 28. Resist mask region 30c defines an upper capacitor plate in second polysilicon layer 28.
With resist mask regions 30a, 30b, and 30c in place, second polysilicon layer 28 is etched to form resistors 32 and 34 over a field oxide region 20 and upper capacitor plate 36 over lower capacitor plate 26. Resist mask regions 30a, 30b, and 30c are then removed and an interlevel dielectric layer 38 is formed over resistors 32 and 34 and over upper capacitor plate 36 to a thickness of about 300 Angstroms. Dielectric layer 38, like dielectric layer 27, may be formed from either a single oxide layer or alternating layers of oxide, nitride, and oxide. A layer of photoresist is then formed over the structure and patterned to form resist mask regions 40a and 40b. Resist mask region 40a is formed over the center portion 32a of resistor 32 leaving end portions 32b and 32c of resistor 32 uncovered. Resist mask region 40b is formed over an area that will contain PMOS devices.
Areas not covered by resist mask regions 40a and 40b are doped with an N type dopant, for example by implanting arsenic with a dose of 8.0E15 atoms/cm.sup.2 at 135 keV. This forms a pair of N+ source/drain regions 42 for NMOS transistor 44 that are self-aligned with a gate 24 and heavily dopes resistor 34, upper capacitor plate 36, and end portions 32b and 32c of resistor 32. An anneal is then performed at 1000 degrees Centigrade for 30 minutes resulting in the structure shown in FIG. 5. As a result of this N type doping, resistor 34, upper capacitor plate 36, and end portions 32b and 32c of resistor 32 have a low sheet resistance of approximately 50-70 ohms/square.
Resist mask regions 40a and 40b are then removed and a layer of photoresist is formed over the structure and patterned to form resist mask region 46. Resist mask region 46 is formed over resistors 32 and 34, transistor 44, and upper and lower capacitor plates 26 and 36 leaving an area in which a PMOS device will be formed. The areas not covered by resist mask region 46 are doped with an P type dopant, for example by implanting boron with a dose of 4.0E15 atoms/cm.sup.2 at 40 keV. This forms a pair of P+ source/drain regions 48 for PMOS transistor 50 that are self-aligned with a gate 24. An anneal is then performed at 950 degrees Centigrade for 45 minutes resulting in the structure shown in FIG. 6.
Photoresist mask region 46 is then removed and a doped glass layer (MLO) 52 having a thickness of about 10000 Angstroms is then formed on the surface of the wafer and patterned and etched to form windows extending through layer 52 and dielectric layer 38 to expose contact areas on resistors 32 and 34 and upper capacitor plate 36, through layer 52 and dielectric layer 27 to expose contact areas on gates 24 and lower capacitor plate 26, and through layer 52 and gate oxide layer 22 to expose contact areas on N+ source/drain regions 42 and P+ source/drain regions 48.
A metal layer is then deposited over the wafer and patterned and etched to form contacts 54 for contacting gates 24, source/drain regions 42 and 48, capacitor plates 26 and 36, end portions 32b and 32c of resistor 32, and end portions of resistor 34. This results in the structure shown in FIG. 7.
Where a circuit design mandates very accurate capacitors, non-precision resistors of minimum size, and low cost, this prior art process is suitable. This process yields accurate polysilicon capacitors, such as capacitor 37, since heavily doped lower and upper capacitor plates 26 and 36 have low sheet resistance values to minimize the voltage coefficient of capacitance. This process also yields non-precision resistors, such as resistor 32, that consume little silicon area since they are made up primarily of center portion 32a which is lightly doped to exhibit high sheet resistance values. This process is also low in cost since it is simple and requires relatively few steps.
This process does however have drawbacks where a circuit design requires resistors that are very precise. Precision resistors should have low temperature coefficients. This process produces resistors, such as resistor 32, that have high sheet resistances and therefore large negative temperature coefficients and resistors, such as resistor 34, that have low sheet resistances and therefore large positive temperature coefficients. Where this process alone must be relied on to produce a precision resistor, resistors with counterbalancing temperature coefficients can be paired in an attempt to produce a single resistor having a low temperature coefficient. By pairing a negative temperature coefficient resistor, such as resistor 32, with a positive temperature coefficient resistor, such as resistor 34, the resulting combination resistor should have a low temperature coefficient. In practice, however, the added design complexity for such combination resistors, inaccuracies resulting from the fact that two resistors are required, and the fact that the effect of possible process variations on the two resistors and their interaction are difficult to predict make this approach cumbersome.
One known approach for dealing with this problem is the use of thin-film resistors as precision resistors. Thin-film resistors provide low temperature coefficients over a wide range of sheet resistances. In order to fabricate a thin-film resistor, an additional deposition, pattern, and etch would have to be performed. Typical materials for such resistors are Nichrome, tantalum, or Cermet (Cr--SiO). A drawback to this solution is the high cost of the complex, additional processing required for thin-film resistor fabrication.
Another known approach for dealing with this problem is to initially dope the second layer of polysilicon moderately instead of lightly with the result that portion 32a of resistor 32 has a moderate sheet resistance (approximately 160 ohms/square) and therefore a low temperature coefficient desired for a precision resistor. A drawback to this approach is that good temperature performance is achieved at the cost of silicon real estate since resistors 32 take up considerably more silicon area when their sheet resistance is moderate as opposed to when their sheet resistance is high. Furthermore, since all resistors have this moderate sheet resistance (or a lower sheet resistance if the resistor is later implanted with the N+ source/drain implant), all large-valued resistors will increase in size not just those needing the low temperature coefficient of resistance.
Accordingly, a need exists for an integrated circuit device having accurate capacitors, non-precision resistors of high sheet resistance, and low temperature coefficient precision resistors that can be fabricated with a double-level polysilicon process without additional, complex process steps.